Problem: The perimeter of a rectangle is 48.  What is the largest possible area of the rectangle?
Explanation: Let $x$ and $y$ be the dimensions of the rectangle.  Then $2x + 2y = 48,$ so $x + y = 24.$  By AM-GM,
\[24 = x + y \ge 2 \sqrt{xy},\]so $\sqrt{xy} \le 12,$ which means $xy \le 144.$

Equality occurs when $x = y = 12,$ so the largest possible area of the rectangle is $\boxed{144}.$